Polyimide films are frequently used as a protective overcoat for semiconductor devices. Manufacturers commonly apply a polyimide layer to the top of a semiconductor device. This may be done before grinding, dicing, and final packaging of the semiconductor device. The protective overcoat may function as a stress buffer and as an attenuation layer for soft error rate (SER) reduction. The protective overcoat generally protects the delicate device components from damage during handling and moisture penetration.
In a semiconductor device, it is preferable for adjacent layers to have good adhesion to one other. For example, when coating a material such as a copper metallization, adhesion between the dielectric and the copper may be a reliability concern. Localized fracture of the dielectric film near a conducting line may lead to catastrophic failure of the chip. Generally, one cause of fracture is stress arising from a mismatch in coefficients of thermal expansion combined with weak interfacial adhesion.
These problems generally are compounded when low dielectric constant (low-k) materials are used in the device. Recent advances in semiconductor device processing technology are requiring the increased use of low-k (k less than about 3) insulating materials in, for example, intermetal dielectric (IMD) or interlevel dielectric (ILD) layers in multilayer devices. Because the mechanical strength of low-k materials is typically less than that of oxides and glasses, stress between the polyimide layer and the underlying low-k layer or layers may be particularly problematic. This stress may cause fracture, delamination, and cracking in the layers of the semiconductor device.
To address these problems, manufacturers frequently apply polyimide as a protective overcoat upon a device having low-k ILD layers. The overcoat may be formed over the final ILD and metal layers. There may be a passivation layer, e.g., silicon dioxide and/or silicon nitride, formed on the device before formation of the overcoat. Polyimide film formation typically includes such well known processes as polyimide coating, polyimide hardening, resist coating, exposure to light, selective removal of the resist layer for patterning, selective etching of the polyimide film using the resist layer as a mask, and removal of the resist layer. Newer, photo-sensitive, polyimide can be patterned directly without using a resist layer.
As stated earlier, stress arising from a mismatch in coefficients of thermal expansion combined with weak interfacial adhesion can be a major problem with low-k materials. When polyimide film is cured, the film typically shrinks resulting in an extra buildup of tensile stress at corners. The extra tensile stress build-up due to polyimide film shrinkage at corners easily induces layer de-lamination. Localized fracture or delamination of the dielectric film near a conducting line may lead to catastrophic failure of the chip.
A solution to these and other problems is desired because of the rapidly increasing use of low-k materials within the semiconductor industry.